This invention relates generally to piezoelectric and pyroelectric polymers and more specifically to a process for treating vinylidene fluoride polymers in order to enhance their piezo and pyroelectric properties.
Certain polymers, including those of vinylidene fluoride, can be formed into films which are highly piezo- and pyroelectric. The existence of at least three crystalline phases (.alpha.,.beta. and .gamma.) in poly(vinylidene fluoride) (PVDF) polymers has been reported. Films containing the .beta. and/or .gamma. phases have the highest activity. It has been shown that a large thermally stimulated current is characteristic of .gamma. phase containing films, i.e., high pyroelectric activity results. The methods used to obtain the .gamma. phase of PVDF have included recrystallization of a melt or recrystallization from dimethyl sulfoxide (Cortili and Zerbi, Spectrochim Acta, 23 A, p. 2216, 1967) crystallization from a monochlorobenzene-dimethyl formamide mixture followed by filtration, vacuum drying for two weeks and annealing at 185.degree. C. for 20 hours (Osaki & Ishida, J. Polymer Sci, Polymer Phys. Ed., 13, p. 1071, 1975); inducing a time-temperature dependent crystal transition (Prest & Luca, J. Appl. Phys., 46, (10), p. 4136, 1975); and washing PVDF in a methanol solution of a siloxane-oxyalkylene followed by pressing into a film which is slowly heated through the .alpha. phase to give a high .gamma. phase content (Prest et al., 175th A.C.S. Mtg. 3/12-17/78, Los Angeles, Organic Coating and Plastics Chemistry Preprints, 38, p. 334 and Prest et al., J. Appl. Phys., 49, (10) p. 5042, 1978; and Prest et al., U.S. Pat. No. 4,095,020). These methods are cumbersome and have disadvantages such as the presence of residual solvent, equipment cost and long processing times. I have now discovered a process for forming increased .gamma. phase containing PVDF materials having enhanced piezo- and pyroelectric activity from .alpha. phase containing PVDF. The process requires no solvents, can be carried out in conventional commercial equipment and can be easily integrated with existing equipment and procedures for forming piezo- and pyroelectric elements.